Abstract

Thermal energy storage is of great interest both for the industrial world and for the district heating and cooling sector. Available technologies present drawbacks that reduce the margin of application, such as low energy density, limited temperature range of work, and investment costs. Phase transition is one of the main phenomena that can be exploited for thermal energy storage because of its naturally high energy density. Constant-volume vapor-liquid transition shows higher flexibility and increased heat transfer properties with respect to available technologies. This work presents a description of the behavior of these types of systems. The analysis is carried out through a generalized approach using the Corresponding State Principle. Variation of internal energy as a function of temperature over a fixed range is calculated at constant volume at different values of specific volume. It is shown that, for lower specific volumes, larger temperature ranges of work can be achieved without occurring in the steep pressure increase typically given by the expansion of liquid. Maximum operating temperature range is increased by up to 20% of the critical temperature with minimal energy loss. In optimal subsets of these ranges of temperature, the energy storage capacity of vapor-liquid systems increases at lower volumes, with energy storage capacity increasing to up to 40% with a 50% increase of the reduced volume. This is especially valid for more complex fluids, which are more interesting for these applications because of their higher heat capacity.

Highlights

  • The development of energy saving technologies is a key factor for the deployment of sustainable processes

  • The analysis done in this work considers two-phase constant volume processes through a generalized approach

  • The increase of pressure of constant volume processes after countercondensation leads to an increase in pressure of up to two orders of magnitude with respect to processes at volume higher than critical;

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Summary

Introduction

The development of energy saving technologies is a key factor for the deployment of sustainable processes. Thermal energy storage systems have been widely investigated because of their role in primary energy saving [1]. The interest for these technologies arises from a number of reasons, such as the need to cover the mismatch between the energy supply and the energy demand or the possibility to exploit a cyclic thermal load that could be present in a process [2]. The applications of these technologies are limited by the low conductivity of these solid-liquid phase change materials, which results in poor heat exchange performances [5, 6]. Vapor-liquid phase change is expected to show much better heat exchange performances with respect to solid-liquid phase change

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